Single action potentials and subthreshold electrical events imaged in neurons with a fluorescent protein voltage probe

Abstract

Monitoring neuronal electrical activity using fluorescent protein-based voltage sensors has been limited by small response magnitudes and slow kinetics of existing probes. Here we report the development of a fluorescent protein voltage sensor, named ArcLight, and derivative probes that exhibit large changes in fluorescence intensity in response to voltage changes. ArcLight consists of the voltage-sensing domain of Ciona intestinalis voltage-sensitive phosphatase and super ecliptic pHluorin that carries the point mutation A227D. The fluorescence intensity of ArcLight A242 decreases by 35% in response to a 100 mV depolarization when measured in HEK293 cells, which is more than five times larger than the signals from previously reported fluorescent protein voltage sensors. We show that the combination of signal size and response speed of these new probes allows the reliable detection of single action potentials and excitatory potentials in individual neurons and dendrites.

Figure 1. The A227D mutation increases the fluorescence response magnitude of CiVS-based voltage sensors containing ecliptic pHluorin or super ecliptic pHluorin

A) Changes in fluorescence intensity in HEK 293 cells expressing either CiVS-ecliptic pHluorin (black, n=9 cells; 10 trials for each cell) or CiVS-ecliptic pHluorin A227D (red, n=6) in response to +100 mV depolarizing pulses (−70 mV holding potential). The lighter traces (grey and pink) indicate the standard error of the data. B) The fractional fluorescence change % ΔF/F (mean ± SEM) produced by +100 mV depolarization steps in seven FP voltage sensors with different FPs inserted at S249 of the CiVS: ecliptic pHluorin (black; n=9), ecliptic pHluorin A227D (red; n=6), eGFP (black; n=10), eGFP A227D (black; n=5), super ecliptic pHluorin (black; n=9), super ecliptic pHluorin A227D (ArcLight, red; n=8), the mUKG emission of Mermaid (green; n=25) C) Left: Peak % ΔF/F (mean ± SEM) vs membrane potential and fitted Boltzmann curves for three CiVS-based FP voltage sensors containing different FPs: ecliptic pHluorin (black), ecliptic pHluorin A227D (red) and super ecliptic pHluorin A227D (ArcLight, pink). Right: Boltzmann-fits of normalized fluorescence change of the three probes. D) Bottom: The fluorescence change of super ecliptic pHluorin A227D (ArcLight) during depolarization (left) and repolarization (right) using a +100 mV depolarization step from −70 mV (black, single trial) and a double exponential curve fit (red). Top: Residual vs time for the on and off fitted curves.

Figure 2. The modulatory effect of the A227D mutation is dependent on a negative charge at that position and on other surface residues of the FP. A larger response magnitude was achieved when the FP was relocated closer to the S4 domain of the CiVS

All responses are produced by +100 mV voltage steps in HEK 293 cells. A) Left: The 3D structure of eGFP (PDB identifier 1EMG) illustrating the position of residues in super ecliptic pHluorin (ArcLight) that were mutated to those found in eGFP: Q80 (cyan), S147 (magenta), N149 (yellow), S202 (blue), Q204 (orange), A206 (green). The A227D mutation is marked with red. Right: The fractional fluorescence change, % ΔF/F (mean ± SEM), of seven CiVS-based voltage sensors containing different mutations of super ecliptic pHluorin inserted at S249 of the linker. These modifications were mutations of super ecliptic pHluorin (A227D) at the sites indicated in the left panel. ArcLight (red, n=10), ArcLight with single point mutations: R80Q (cyan, n=6), D147S (magenta, n=7), Q149N (yellow, n=6), F202S (blue, n=8), T204Q (orange, n=8) and T206A (green, n=6). A single trial was recorded from each cell and the peak values of these trials were averaged. B) Left: Averaged optical traces of CiVS-ecliptic pHluorin and seven mutant probes carrying different point mutations at residue 227 of the FP. CiVS-ecliptic pHluorin (A, grey, n=9). Point mutations at residue 227: A227D (red, n=6), A227E (pink, n=8), A227K (dark blue, n=4), A227R (intermediate blue, n=8), A227H (light blue, n=6), A227Q (green, n=5), or A227N (light green, n=9). Ten trials were averaged for each cell and then the values from the different cells averaged. Right: Summary of the peak % ΔF/F (mean ± SEM) for probes in the left panel. C) Left: Averaged optical traces of ArcLight and ArcLight-derived probes with super ecliptic pHluorin A227D moved to different locations within CiVS: ArcLight Q239 (purple, n=7), ArcLight M240 (blue, n=8), ArcLight K241 (cyan, n=7), ArcLight A242 (green, n=7), ArcLight S243 (orange, n=7) and ArcLight (S249; red, n=10). A single trial was recorded from each cell and the optical traces of these trials were averaged. Right: Summary of the peak % ΔF/F (mean ± SEM) for probes in the left panel.

Figure 3. Detection of action potentials in neurons with the ArcLight A242 probe

A) Sample traces of single trial recordings of spontaneous action potential bursts from the neuron shown in panel B. All traces have double exponential subtraction of bleach and were low pass filtered with a Kaiser-Bessel 30 filter (200 Hz cut off). B) An 80×80 depixelated image of the neuron presented in A. Scale bar: 50 µm. C) The optical and voltage recording of a stimulated action potential burst from a different neuron expressing the ArcLight A242 probe. All traces have double exponential subtraction of bleach and used a Binomial smoothing of one and a τ of four.

Figure 4. Detection of subthreshold depolarizations in neurons with the ArcLight Q239 probe

A) Sample traces of single trial recordings of spontaneous subthreshold and action potential activity from the neuron shown in panel B. All traces have double exponential subtraction of the bleach and are low pass filtered with a Kaiser-Bessel 30 filter (200 Hz cut off). The arrowheads indicate the depolarizations (likely EPSPs) that triggered visible changes in the optical recordings. B) An 80×80 depixelated image of neuron presented in A. Scale bar: 50 µm.

Figure 5. Detection of subthreshold events and action potentials in the soma and dendrites of a neuron with the ArcLight Q239 probe

A) Sample traces of single trial recordings of action potential (left panel) and subthreshold events (right panel) from four locations of the neuron in panel B. Optical recordings (colored): % ΔF/F from the area of interest in the same color in panel B; soma electrode voltage recordings (black). All traces have double exponential subtraction of the bleach and are low pass filtered with a Kaiser-Bessel 30 filter (200 Hz cut off). B) An 80×80 image of a neuron presented in A with the regions of interest averaged to produce the traces seen in A. Scale bar: 50 µm.